There is widely known a scanning electron microscope which obtains a two-dimensional image of a scanning area by two-dimensionally scanning an electron beam probe converged on a specimen, detecting signal electrons generated at an irradiation point, and mapping signals relating to the amount in synchronization with scanning of a primary electron beam.
Signal electrons are generally classified into secondary electrons and backscattered electrons according to their energy. The backscattered electron refers to an electron which enters the specimen, repeats elastic scattering and inelastic scattering, and is consequently emitted from the surface of the specimen. Thus, a backscattered electron has a peak in its yield at the approximately same energy level as that of an incident electron. On the other hand, the secondary electron refers to an electron which is emitted from the surface of the specimen among low-energy electrons generated by the backscattered electrons when they cause the inelastic scattering. Thus, the secondary electron has a peak in its yield at an energy level in the order of a few eV. In general, a signal electron with 50 eV or less energy is referred to as the secondary electron, which is differentiated from the backscattered electron.
A compositional difference is observed as a contrast in a detection image of the backscattered electrons because the yield of the backscattered electrons depends on an average atomic number of the specimen. Furthermore, it is known that a channeling contrast, observed in a specimen having the one and same composition when a crystal orientation is partially different on the surface of the specimen or when a crystal defect is included, also derives from the backscattered electrons. These contrasts need to be detected by separating the backscattered electrons from secondary electrons.
On the other hand, there is recently increasing significance of a low-acceleration observation with an accelerating voltage of about 3 kV or lower for the purpose of an avoidance of damaging and charging of the specimen due to irradiation of a primary electron beam as well as an observation of the top surface of the specimen. Although the scanning electron microscope used at low acceleration employs an observation technique of retarding the primary electron beam immediately before the specimen in order to minimize an aberration exposed in a low-acceleration, when the retardation method is used, it is difficult to detect the secondary electrons and backscattered electrons separately because the secondary electrons are accelerated to the energy level similar to that of the backscattered electrons in consequence of an electric field created around the specimen. However, providing an energy barrier, a Wien filter, or the like on a trajectory of the signal electrons makes it possible to selectively detect the secondary electrons and backscattered electrons. The following Patent Documents 1 to 3 are reported as known means of selectively detect the signal electrons in the low-acceleration region of 3 kV or lower accelerating voltage using a scanning electron microscope.